Lets say making a Marine creates an ability.
and making two Marines creates another ability,
^ how do I do that?

Please give more specifics. What kind of map is it? What exactly do you mean by marines creating abilities? Does the player have to put two marines in a location to buy/use an ability?

Pretty simple, really. See attached.

Basically, you have 5 in-game seconds (or whatever you want) to create however many marines. I only triggered it for one and two.

Attachments:

5 ingame seconds = 40 death counters (with hypers)


So you have an event that starts the counter (the time you have to cast your spell):
C: [your event]
A: Set deaths of 'counter' to 1


And another trigger always counts the counter up if it's been initialized (set to 1 or higher):
C: Deaths of 'counter' at least 1
A: Add 1 to deaths of 'counter'


And then a set of triggers that detects the spell combo when the time is over:
C: Deaths of 'counter' at least 40
C: Current Player brings exactly 1 Marine the 'Spell Area'
A: Set deaths of 'counter' to 0
A: [Execute Spell 1]

C: Deaths of 'counter' at least 40
C: Current Player brings exactly 2 Marines the 'Spell Area'
A: Set deaths of 'counter' to 0
A: [Execute Spell 2]

Nude's triggers are a good base to work from, but you may want the 2nd spell to fire immeditaely should they bring 2 marines to the location, rather than waiting for the 5 second period to end. Also nude's trigger won't fire if they create 3 or 4 marines (could be do-able, depending on your build time and the combo 'window' you're using).

Suggest:
C: Deaths of 'counter' at least 1
C: Current player brings at least 2 Marines to 'Spell Area'
A: Set death of 'counter' to 0
A: [Execute Spell 2]

But really combos aren't great, simply because it slows gameplay down. You'll see from Nude's triggers that the player must always wait 5 seconds before they can cast spell 1 or 2. Using my trigger, you can cast spell 2 very quickly but must wait at least 5 seconds to cast spell 1. This could potentially be used as a facet of gameplay; although generally you'd make a spell that required 2 units to cast more complex/powerful than one that only required 1 unit, so it'd be a bit counter-intuitive.

Of course if you have a very short window, say 1-2 seconds, and fast build times on your units (must be at least 1: using 0 will crash) then this may not have too much of an impact on your gameplay. Alternatively you may be better off simply using single units from the Barracks and assing spell 1 to Marine and spell 2 to Firebat, that way each spell can be cast immediately without having to 'combo'.

With this, sure it may fire immediately, but how would you make spell one and spell two both fire immediately without any sort of timer or switches being set (death counters too)? If spell one fired immediately, you would never be able to satisfy spell two's conditions, because the one marine would already have been used for spell one.

As for creating more than 2 marines, there are numerous ways to fix that. Limiting PSI, limiting build space, gas required for marines, etc.

I'm not sure if Nude thought I was the OP or not, but we both created nearly identical triggers .

Lanthanide is talking about making the spell cast instantly after the SECOND marine is created, not after the first is created.

Awesome. Thanks a bunch

The obvious limitation is in the inability to cast your first spell instantly. But there is no reason to make the delay five in game seconds.

Instead, (and as per this thread) build times are measured in frames. Therefore a build time of one should be one frame. To be safe, make the cutoff time 2 DCs (4 frames). To cast the l1b spell, the player must queue the units by rapidly pressing the key twice. If the 2 DC time isn't working, try going up by 1 DC until it does.

I'd suggest that SC probably isn't responsive enough to give instant 1-frame training times for units. Easy way to test: make a map with a 1 frame training rate for a unit, select the factory and spam the hotkey to build that unit. Do this for 5 real time seconds and see if the number of units produced matches what you calculate it should be. You could even add in a continual "move unit" trigger action to move them away from the factory so that you don't get placement / building exit blocked areas, and do the test for 10-20 seconds for better accuracy.